The invention relates to printing methods and apparatus which employ inks curable by radiant energy. The methods and apparatus are adaptable for use with ink jet print heads and thermal transfer print heads.
In conventional printing processes and apparatus which employ inks curable by radiant energy, particularly those which employ ink jet print heads and thermal transfer print heads, it is difficult for light or other radiant energy to penetrate colored ink layers. The pigments and dyes block or absorb the radiant energy, reducing the energy available to initiate the polymerization and often resulting in surface cure. The degree of cure decreases with increasing depth within the ink layer. It is desirable to provide a method and apparatus which will more thoroughly cure radiation curable inks with such pigments and dyes.
Ink jet printers are likely to find increased use as the technology is advanced due to high print speeds and economy of operation. Typical ink jet printers operate by employing a plurality of actuator elements to expel droplets of ink through an associated plurality of nozzles. A typical print head actuator is described in U.S. Pat. No. 4,516,140. Each actuator element is typically located in a chamber filled with ink supplied from a reservoir. Each actuator element is associated with a nozzle that defines part of the chamber. On energizing a particular actuator element, a droplet of ink is expelled through the nozzle toward the print medium either by pressure or vaporization. Those wherein the ink is drawn through the nozzle by an electrostatic field are also known. In most configurations, the print head containing the nozzles is moved repeatedly across the width of the print medium on guide rails. After each movement, the print medium is advanced to the width of the swath for the next pass of the pen. In some configurations, such as in high volume printers, the print head extends across the full width of the print medium. At a designated number of increments, each of the nozzles are caused to either eject ink or refrain from ejecting ink. The movement over the medium can print a swath approximately as wide as the number of nozzles arranged in the column.
These conventional ink jet printing methods and apparatus have limitations. Such printing methods often suffer in definition and quality and, in some cases, print speed due to the low viscosity required of the ink employed. Low viscosity values for the ink are required to enable ejection from the print head. This often necessitates high solvent content. The high solvent content requires that the ink be drawn into the print medium to enable rapid drying. When such inks are drawn into the print media, the edges tend to become less defined. Inks of different colors may bleed into each other when drawn into the absorptive print media. Where the print media has a low absorption rate for the ink, such as transparency film, the ink tends to clump together due to surface tension. In addition, print speed is reduced since the ink takes a longer time to dry on the non-absorptive substrate. Another problem which may be encountered in ink jet printing is paper cockle, wherein the print media swells once the ink is absorbed, causing the paper to warp (cockle). It is desirable to provide an ink jet printing method and apparatus wherein the print quality and print speed do not suffer from the high solvent content of the ink.
In thermal transfer printing, images are formed on a receiving substrate by heating extremely precise areas of a print ribbon with thin film resistors. This heating of the localized area causes transfer of ink or other sensible material from the ribbon to the receiving substrate. The sensible material is typically a pigment or dye. These techniques provide images with higher definition and quality than ink jet methods in that high viscosity inks are used which need not wick into the receiving medium. Print speed is not delayed by wicking of the ink. However, UV curable inks and visible light curable inks suffer from surface cure because of limits on penetration of radiant energy.
It is an object of the present invention to provide a printing process and apparatus for radiant energy cured ink wherein surface cure is reduced or eliminated.
It is another object of the present invention to provide a printing process and apparatus for radiant energy cured ink wherein cure of the ink is accelerated.
It is an additional object of the present invention to provide a printing process and apparatus for radiant energy cured inks which employ inks with a high solvent content wherein wicking of solvent in the print medium is reduced or eliminated.
It is a further object of the present invention to provide a printer which employs an ink jet print head and provides printed images with improved definition and quality at high speeds.
It is an additional object of the present invention to provide a printing process and apparatus which employ an ink jet print head and radiation curable inks which form ink images with improved definition and quality.
Upon further study of the specification, drawings and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
The above objects are achieved through the method and apparatus provided by this invention. In the method aspect of this invention, there is provided a two-stage printing process for radiant energy cured ink which comprises depositing such an ink in the form of an image on the surface of an intermediate substrate. The ink image formed has an upper surface and a lower surface. The upper surface of the image on the intermediate substrate is exposed to radiant energy to at least partially cure the ink image. This exposed ink is then transferred from the intermediate substrate to a receiving substrate, wherein the image and its surface are reversed to form a transferred ink image. This transferred ink image on the receiving substrate is then optionally exposed to radiant energy to further cure the ink.
In preferred embodiments, the ink is deposited on the intermediate substrate by an ink jet print head, although use of other printing techniques, such as thermal transfer printing, is also suitable. In addition, in preferred embodiments, the radiant energy is ultraviolet light; however, the use of visible light or infrared radiation is also suitable. In curing the ink both before and after transfer to the receiving substrate, both sides of the ink can be cured, allowing greater penetration of radiation and elimination of single surface cure.
There is also provided a variation of this process wherein the ink is deposited on an intermediate substrate, exposed to radiant energy and transferred to a receiving substrate as described above but the ink image is also heated to a temperature above the ambient temperature on the intermediate substrate and the transferred image cooled to ambient temperature. In this method, exposing the transferred ink image on the receiving substrate to radiation is optional.
In another aspect of this invention, there is provided a printer which comprises an image forming member adapted to form images with an ink curable by radiant energy, an intermediate substrate positioned to receive ink images from said image forming member, an advancing mechanism for moving the surface of the intermediate substrate relative to the image forming member, a radiant energy source positioned to expose the ink images on the surface of the intermediate substrate, a feeding mechanism for receiving substrates and transferring the ink image to a receiving substrate and a second radiant energy source to expose the transferred ink image to radiant energy.
There is also provided a variation of this printer where the image forming member is an ink jet print head. There is included a means for heating the ink image on the intermediate substrate above ambient temperature. With such devices, the second radiant energy source is optional.